1. Field of the Invention
The invention is directed to an improved method for controlling or regulating an electronically triggerable vehicle brake system operating on the return principle, and to an improved electronically controlled vehicle brake system operating on the return principle.
2. Description of the Prior Art
An electronically controlled vehicle brake system, operating on the return principle, is known for instance from German Patent Disclosure DE 199 40 263 A1. This known vehicle brake system has a master cylinder, actuatable by pedal by the driver, with two brake chambers to each of which one brake circuit is connected. In each brake circuit there are two wheel brakes as well as two devices for modulating the brake pressure at these wheel brakes. Each device for modulating the brake pressure includes one pressure buildup valve and one pressure reduction valve. The pressure buildup valve is connected into a first pressure fluid connection or line between the master cylinder and the wheel brake, while the pressure reduction valve is disposed in a second pressure fluid connection or line, which carries pressure fluid away from the wheel brake. An externally driven pressure generator is also present. Its intake side is coupled downstream of the pressure reduction valve to the second pressure fluid connection that carries pressure fluid away, while its intake side is connected upstream of the pressure buildup valve to the first pressure fluid connection and to the master cylinder.
For regulating the brake pressure in one of the wheel brakes, there is an electronic control unit. It triggers the pressure buildup valve, the pressure reduction valve, and the pressure generator in accordance with the instantaneous slip ratios at the wheel brakes of a vehicle. When wheel slip is ascertained, that is, if a wheel is tending to lock, the pressure generator pumps pressure fluid out of the associated wheel brake away via the pressure reduction valve, until the brake pressure has finally been reduced to such an extent that there is no longer any risk of locking. During this time, the pressure buildup valve is closed. This operating principle is called the return principle.
In such return vehicle brake systems, in the active mode, that is, when the pressure generator is triggered, this pressure generator is driven with an at least intermittently constant rpm. In the event that at least the brake pressure of one vehicle wheel of one brake circuit is above the set-point pressure, then as explained, pressure fluid is fed back out of the affected wheel brake into the brake fluid chamber of the master cylinder via the opened pressure reduction valve. This results in either a brake pedal that moves backward counter to the actuation direction or, via the floating piston of the master cylinder, a stream of pressure fluid into the second brake chamber. In an ensuing new pressure buildup phase, that is, the next time one of the pressure buildup valves is opened, the brake circuit is again supplied with pressure fluid from the master cylinder. Because of the varying quantities of pressure fluid in the master cylinder as a result, pedal motions occur, which the driver perceives as annoying. In addition to the mechanical feedback at the brake pedal, unwanted noise is produced. The relatively low frequency with which these pedal motions occur must be distinguished from the high-frequency pedal motions caused by the intermittent mode of operation of the radial piston pumps that are typically used as the pressure generator.